Interferometers can be employed in optical systems which, by way of example, are capable of measuring spectral characteristics of light sources or one or more of the positional coordinates of distant points relative to a known frame of reference. With respect to such systems, temperature changes and mechanical vibrations in the operative environment can cause undersirable variations in the spacing between the two optical plates which comprise the interferometer. As a result, the accuracy of the measurements made by such optical systems is adversely affected.
To date, several different techniques have been devised to monitor and correct the aforementioned problem. All of such techniques entail the use of some type of servo system which mechanically adjusts the position of the optical plates which comprise the interferometer in response to a detected, undesirable variation in the spacing therebetween. As can be readily appreciated, such servo systems typically comprise an arrangement of a plurality of components which require a high degree of calibration, and which are inevitably susceptible to mechanical failure.
In one type of optical measuring system, modulatory means can be employed to provide a point optical radiation source at one or more distant points whose positional coordinates are to be determined. In addition, such a system can employ a multiple-beam interferometer means (e.g. Fabry-Perot interferometer) as an angular filter so as to limit the direct transmission of incident optical radiation emitted from each aforementioned point optical radiation source to those radiation beams which encounter the front optical plate of the interferometer means at substantially a particular angle of incidence, referred to as the operative angle of acceptance. With respect to this type of a system, it may be preferable to maintain, or stabilize, the operative angle of acceptance of the interferometer at a relatively constant and predetermined value in order for accurate measurements to be obtained. A system of this nature is described in a co-pending application assigned Ser. No. 646,638 now U.S. Pat. No. 4,627,722.
Where the maintenance of a relatively constant and known value for the operative angle of acceptance of a multiple-beam interferometer means in the aforementioned type of optical system is desired, environmental temperature changes and vibrations can create significant problems since such temperature changes and vibrations can, as noted above, cause undesirable variations in the spacing between the two optical plates of the interferometer means. Such temperature variations can also cause a variation in the refractive index of the optical plates. Both of the aforementioned types of variations can cause the operative angle of acceptance of the interferometer means to deviate from the desired value therefor. Although a mechanical servo system could perhaps be applied to stabilize the operative angle of acceptance by controlling the spacing between the optical plates of the interferometer means, the aforementioned drawbacks discourage the application of the same. In addition, the application of known mechanical servo systems cannot presently account for the effect of temperature variations upon the refractive index of optical plates comprising an interferometer.